webclient and expect100continue

Question

What is the best way to set expect100continue when using WebClient(C#.NET). I have this code below, I still see 100 continue in the header. Stupid apache still complains with 505 error.

        string url = "http://aaaa.com";
        ServicePointManager.Expect100Continue = false;

        WebClient service = new WebClient();           
        service.Credentials = new NetworkCredential("username", "password");
        service.Headers.Add("Content-Type","text/xml");

        service.UploadStringCompleted += (sender, e) => CompleteCallback(BuildResponse(e));
        service.UploadStringAsync(new Uri(url), "POST", query);

Note: If I put the above in a console app and let it run - then I do not see the headers in fiddler. But, my code is embedded in a user library which is loaded by a WPF app. So, Is there more to Expect100Continue in terms of thread, initialization, etc. Now, I think it is more of my code issue.